According to the report of United Nations, in 2010, data show that anthropogenic mercury emission generated by industries related to natural coal, mining, cement, steel and alkali chlorine shares a ratio up to 80 percents (%) of total global mercury emission, which is the largest source of mercury emission in the global environment. U.S. Environmental Protection Agency has announced mercury together with other toxic pollutants as prior control targets. The other nations like Taiwan, the EU, China and India also have control regulations, for mercury is one of the crucial indicators of world's first-class pollutants. The use of mercury-removing technologies, like combustion, gasification, etc., for fossil fuels becomes an important issue of global air pollution.
Main sources of mercury are emission from including fixed coal-fired power boilers, cogeneration boilers, cement kilns and city incinerators. Industrial mercury-containing wastes are mainly generated from coal industry, which may enter into environment through air and is bound to be regulated.
Modern mercury-removing technologies use activated-carbon bed or activated carbon injection (ACI) in flue gas for removing mercury. These technologies are operated below 100 celsius degrees (° C.) with shortcomings like low mercury concentration, high equipment cost, high energy consumption and non-regeneration. In 2008, a prior art was revealed that, by using a middle-aperture silicon substrate, mercury could be removed at 150° C. Another prior art is a wet mercury-removing technology, which usually uses equipments of selective catalytic reduction (SCR) combined with flue gas desulfurization (FGD) and is also a flue-gas mercury-removing technology. Because more energy is consumed for cooling and the desulfurized product would be combined with mercury, this technology may cause environmental pollution. Therefore, gasification-related technologies are currently under developed. Advantages of these gasification-related technologies include higher concentration, smaller equipment, higher equivalence and reuse. Besides, trace elements in dust and other wastes, which may cause secondary pollutions, are prevented from being generated during production. Toxicity of catalyst (like which comes from water gas shift reaction) is also avoided. These gasification-related technologies can be operates at 200° C. with more than 90% mercury-removing efficiency achieved.
The main concern of these technologies is to enhance the temperature up to 150˜450° C. on using mercury-removing reagent. In the past, precious metals and rare-earth metals are usually used. For example, the U.S. NETL laboratory developed a palladium-aluminum (Pd—Al) material; Spain CSIC developed a gold-containing activated-carbon (Au-AC) material; and China developed cerium-dioxide-containing activated-carbon (CeO2/AC), etc. However, mercury-removing technologies using precious metals and rare-earth metals have higher costs; and some metals, such as platinum (Pt), Pd, ruthenium (Ru), Au-AC, and CeO2/AC, may easily cause recession in an oxidizing environment (O2, HCl, SO2 . . . ). Recently, transition metals are used, which are mostly impregnated in carriers, including Cu/HZSM-5 (Fuel Processing Technology 104 (2012) 325-331), MnO2/Monoliths (Fuel 108 (2013) 13-18), and nano-ZnO (Journal of Fuel Chemistry and Technology, 41 (11) 2013, 41 (11): 1371-1377). These transition metals are widely used in industrial denitration catalysts. Their costs are also relatively inexpensive. These mercury-removing reagents are made by being attached to carriers, where single load of metal oxides is typically less than 50 wt % and has a greater difference between different types of metal oxides. As a result, adsorption capacity cannot be significantly improved in a fixed bed; and, surface abrasion in a fluid bed is also obvious.
Another prior art is form synthetic mercury-removing reagent into nano-particles for increasing surface areas, where, typically, a surface area bigger than 50 m2/g must be added with more materials through more fabricating processes. Yet, their performance stabilities accompanied with temperature enhanced still need to be tested and evaluated.
Hence, the prior arts do not fulfill all users' requests on actual use.